Bug 29381: Prevent sending of automatic renewal messages on every cron run

Bug 29381: Prevent sending of automatic renewal messages on every cron run

Currently the auto-renewal digest messages are sent on every cron run
even if there was nothing to renew or no renewal errors.

This regression was introduced in the commit "Bug 18532: Add
individual issues to digest notice and hide auto_renewals messaging
preference when not needed".

To test:
 1) set syspref AutoRenewalNotices to be according to patron
    preferences
 2) Enable renewal digest messages on a patron's messaging preferences
 3) Checkout a book for patron, during the checkout use the Checkout
 settings menu to check the box "Automatic renewal"
 4) Run
    $ perl misc/cronjobs/automatic_renewals.pl --send-notices --confirm --digest-per-branch
    $ perl misc/cronjobs/automatic_renewals.pl --send-notices --confirm --digest-per-branch
 5) Notice you have now two renewal messages for the patron
 6) Apply patch
 7) repeat step 4) and notice you don't get anymore these unnecessary
    renewal messages

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>